In the context of cold weather aeronautics, it is necessary to provide an accurate assessment of whether a surface of an aircraft is bare, or covered with a layer of solid and/or liquid deposits such as ice and water, and to quantify the deposits so that an appropriate measure can be taken to ensure safe use of the aircraft. Typically this has been accomplished by visual or tactile inspections, however this is undesirable for numerous reasons, including gaining access to the surfaces (if the surface is moving, the area is dangerous, etc.), time and expense, etc. In other situations a like evaluation of surfaces on building structures, roadways, etc. is needed. Further in marine environments icing and its detection may pose particular problems given inclusions of brine in the ice, which make thickness of the layer difficult to quantify. It is frequently desirable to determine not only the presence but also the thickness of an ice layer.
A variety of techniques for transparent ice thickness measurement are known. Many known techniques use non-remote (i.e. contact) measurements, including techniques that embed emitters, sensors or other detection enabling devices on surfaces of aircraft. Electrical, acoustic, mechanical and optical devices have been proposed to date for non-remote measurement purposes. All contact type (non-remote) devices and certain optical devices have to be incorporated into or in close contact with, the surface on which the layer is to be detected, e.g. ice on an aircraft wing. The same holds true for their electrical connections that run from the sensor to the data logger. For this reason they are expensive to install and operate. Furthermore there are critical locations on structures, e.g. fuel tanks in wings, where embedded or manipulated sensors/emitters cannot, for practical reasons, be placed. Also, embedded devices generally give information for only the locations where they are placed on the structure (a few at most). Accordingly, there are considerable economic and practical benefits of remote (i.e. non-contact) techniques for ice thickness measurements.
One example of a remote system for ice detection known in the art is U.S. Pat. No. 5,921,501 to Pernick. Pernick teaches the scanning of a surface of an aircraft with a continuous wattage laser beam in a manner whereby the surface scatters the laser beam, the detecting of the laser light scattered by the surface, and the processing of the detected scattered laser light to reconstruct images of the surface, thereby indicating area of ice and water on the surface. The laser beam may have a first wavelength absorbed by either deicing fluid or water and ice and a second wavelength absorbed by the other of either deicing fluid or water and ice. Pernick's system requires separate handling of two signals to determine a composition of a layer. Furthermore absorption may not provide accurate enough an indication of thickness of the layer, in certain instances. Finally it does not appear that Pernick's system can be applied to measure or detect ice other than glaze ice. Accordingly rime ice or frosted glaze ice may not be detected or correctly measured using Pernick's system.
A device based on the degree of absorption of infrared radiation for detection and thickness measurement of ice is described in U.S. Pat. No. 4,808,824 to Sinnar. Sinnar teaches a system for detecting the formation of ice and/or water on a surface and measuring the thickness thereof. The ice detection and measurement system includes a radiation source for providing a discontinuous transmission alternating between a pair of narrow band infrared signals, each centered at a different, predetermined, discrete wavelength. The discontinuous alternating signal is applied to an optical system where it is divided into two beams for application to two respective detectors. A first reference detector includes a photoconductive cell for conversion of one of the two beams into a reference signal for each of the discrete wavelengths transmitted by beam. A second test detector includes a photoconductive cell for receipt of the other beam after transmission through the ice and/or moisture formed on the surface of an infrared transparent cover, for establishing a test signal responsive to infrared radiation absorbed at each of the two discrete wavelengths. The test and reference signals at each of the two discrete wavelengths are compared in microcomputer for detecting and measuring ice accumulation, to distinguish icing, frost and water accumulation, and to monitor progress of an icing/de-icing process.
The effectiveness of this technique would be compromised considerably by the effect of inclusions (air bubbles, grain boundaries etc.) in the ice that influence the intensity of radiation detected by the sensor. Furthermore the absorption qualities of water and ice are limitations on the wavelengths used for the system, and generally require more expensive sources.
MDA of British Columbia, Canada, has an optical system (“Ice-Cam”) that uses spectral reflectivity to detect ice by its unique spectral signature. Unfortunately using a band of wavelengths in spectral reflection or absorption techniques requires more expensive transmission and detection equipment.
Applicant has previously developed a method and apparatus for remote detection and thickness measurement of ice or liquid layer (U.S. Pat. No. 5,400,144) using a laser and sensor (video camera or diode array). That method uses the laser to produce a certain pattern on the surface due to internal reflection of the light within the ice/liquid layer. The image can be processed to give the thickness of the layer. The technique is best suited to diffuse reflecting surfaces and optically clear layers with thicknesses from 500 μm to several centimeters, but can be calibrated for thinner layers. Applicant's previous method was not ideally suited for measurements of less transparent layers such as rime ice.
In a related field, non-contact optical devices have been developed for ice detection and measurement for road surface evaluation. Their range, i.e. distance of device from the surface, is fairly limited, and accordingly they are not applicable to aircraft ice detection and measurement, or for other distance measurements that are performed remotely.
There therefore remains a need for a system and apparatus for remote detection, and measurement of a layer on a surface, such as ice regardless of a degree of transparency of the layer.